Thermodynamic and Magnetic Properties of Superconductors with Anisotropic Energy Spectrum,MgB<Subscript>2</Subscript>

The behavior of the superconducting transition temperature T _c and that of the jump of electron heat capacity (C _S −C _N )/C _N of the compound MgB₂ at T=T _c with substitution of boron and magnesium atoms by other atoms from the periodic table of the elements, corresponding to introduction of additional electrons or holes in this compound are researched. The microscopic superconductivity theory in MgB₂ systems in the magnetic field parallel to the crystallographic axis (H ‖ c) is built. The magnitude of the upper critical field H _c2 is determined and its temperature dependence in two-band systems with different and identical topologies of Fermi surface cavities of the corresponding bands is studied. The obtained results and their comparisons with the experimental data demonstrate that all kinds of anomalies of the physical properties of the compound MgB₂ are effectively described by the two-band model.     

Studies on the Composite System of Bi-2212 Glass Ceramic and MgB<Subscript>2</Subscript> Superconductors

The composites of glass ceramic Bi-2212 and MgB₂ superconductors were prepared at ambient conditions. The transmission electron microscopy images of the composite samples illustrate the presence of glass ceramic inclusions in bulk MgB₂. Temperature-dependent magnetization of the composite samples shows two superconducting transitions: one at 80 K corresponding to the Bi-2212 phase and a second one at 39 K corresponding to the MgB₂ phase, suggesting that the two superconducting phases are separated with clear boundaries. The critical current density (J _c) and pinning force values are increased in composite systems by an order of magnitude compared to that of individual samples. The pinning mechanism in the composite sample is the same as in the matrix phase. Reduced field maxima (h _max) are observed at 0.15 for composite samples. A low value of h _max for composite samples indicates the random orientation of grain boundaries and repulsive pinning force in the composite samples.     

Critical Temperature of Smart Meta-superconducting MgB<Subscript>2</Subscript>

Enhancing the critical temperature (T _C ) is important not only to widen the practical applications but also to expand the theories of superconductivity. Inspired by the meta-material structure, we designed a smart meta-superconductor consisting of MgB₂ microparticles and Y₂O₃/Eu³⁺ nanorods. In the local electric field, Y₂O₃/Eu³⁺ nanorods generate an electroluminescence (EL) that can excite MgB₂ particles, thereby improving the T _C by strengthening the electron–phonon interaction. An MgB₂-based superconductor doped with one of four dopants of different EL intensities was prepared by an ex situ process. Results showed that the T _C of MgB₂ doped with 2 wt% Y₂O₃, which is not an EL material, is 33.1 K. However, replacing Y₂O₃ with Y₂O₃/Eu³⁺II, which displays a strong EL intensity, can improve the T _C by 2.8 to 35.9 K, which is even higher than that of pure MgB₂. The significant increment in T _C results from the EL exciting effect. Apart from EL intensity, the micromorphology and degree of dispersion of the dopants also affected the T _C . This smart meta-superconductor provides a new method to increase T _C .     

Microstructure and Transport Properties of Compaction-Modified In Situ Fe/MgB<Subscript>2</Subscript> Wires

This study has shown the effect of excessive mechanical deformation by pressing and consecutive heat treatment, respectively, on the microstructure and transport properties of mono-core sintered in situ Fe/MgB₂ wires. These wires were pressed into short tapes under 1 GPa uniaxial pressure. Transport properties like critical temperature, transition width and engineering critical current density of the same samples were determined before and after pressing and after annealing. Phase formation and grain size of the samples were studied with XRD. The SEM analysis indicated that the mechanical deformation reduced initial voids to a large extent but caused cracks, increased the number of grain boundaries and introduced weak links. The subsequent heat treatment on the pressed samples improved the microstructure and the grain connectivity. Possibility of a final densification of the reacted in situ wires was discussed in terms of transport properties.     

Monofilamentary In Situ Fe/MgB<Subscript>2</Subscript> Superconducting Wires Fabricated by Pellet-in-Tube Method

Thin monofilamentary Fe/MgB₂ superconducting wires without barriers are investigated by means of electrical transport measurements and surface and structural analysis methods. Small diameter wires are fabricated by pellet-in-tube method (PeIT) to obtain a high uniform initial filling density and heat treated as a function of various sintering temperatures and times. The results are discussed in terms of the grain connectivity, Fe₂B phase formation, and the relation between wire diameter and sintering conditions. We suggest that PeIT has a crucial importance to achieve homogeneous initial filling density, which leads to the fabrication of uniform long-length MgB₂ wires.     

Fluctuation induced conductivity of polycrystalline MgB<Subscript>2</Subscript> superconductor

We report fluctuation-induced conductivity (FIC) of the polycrystalline MgB₂ superconductor in the presence of magnetic field. The results are described in terms of the temperature derivative of the resistivity, dρ/dT. The dρ/dT peak temperature observed for H = 0 Tesla at 39 K remains very distinct under applied fields of 6 Tesla and 8 Tesla at 22 and 20 K respectively. Aslamazov and Larkin (AL) equations are used to explain the anisotropic nature of the polycrystalline MgB₂. The effective coherence length, ξ _p (0) determined experimentally is 55.17 Å, which roughly matches with previously reported experimental work.     

Liquid-Phase Shock-Assisted Consolidation of Superconducting MgB<Subscript>2</Subscript> Composites

An original two-stage liquid-phase hot explosive compaction (HEC) procedure of Mg-B precursors above 900 ^°C provides the formation of superconductivity MgB₂ phase in the whole volume of billets with maximal T _c = 38.5 K without any further sintering. The liquid-phase HEC strongly increases the solid-state reaction rate similar to photostimulation, but in this case, due to the high penetrating capability of shock waves in a whole volume of cylindrical billets and consolidation of MgB₂ precursors near to theoretical density allows one to produce bulk, long-body cylindrical samples important for a number practical applications.     

Effect of nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on formation and superconductivity of bulk MgB<Subscript>2</Subscript>

Bulk materials of MgB₂ have been prepared with the stoichiometry of MgB₂(Al₂O₃) _x (x = 0, 2, 5, 10 and 20% nano-Al₂O₃ powders), by using solid-state reaction route. All samples were sintered at 750 °C for 30 min in a calorimeter to monitor the sintering reaction process. It is found that the onset temperatures of reaction between Mg and B powders increase significantly with increasing the amount of Al₂O₃. However, the reaction time is shortened for the nano-Al₂O₃ powders can effectively activate the reaction as a catalyst. The critical transition temperature decreases from 38.5 to 31.6 K, and the corresponding temperature window becomes narrow (less than 2.6 K). Furthermore, the amount of MgO impurity was found to increase with the increase of Al₂O₃, which probably indicates that partial Mg was replaced by Al.     

Addition of Sb<Subscript>2</Subscript>O<Subscript>5</Subscript> into MgB<Subscript>2</Subscript> Superconductor Obtained by Spark Plasma Sintering

High-density (92–98 % of the theoretical density) MgB ₂ samples added with Sb ₂ O ₅ ((MgB ₂)+ (Sb ₂ O ₅) _x ,x= 0, 0.0025, 0.005, 0.015) were obtained by Spark Plasma Sintering. A higher amount of additive decreases density. In added samples, grains of secondary phases are located at MgB ₂ grain boundaries and they are of large size. Hence, Sb ₂ O ₅ does not promote effective flux pinning, connectivity is lower, and this suppresses the critical current density and the irreversibility field. Pinning force-related parameters indicate that added samples are close to the point pinning region and they show a higher grain boundary pinning contribution when compared with pristine MgB ₂ sample and when temperature is lower. It is speculated that for fixed processing conditions and Sb-oxide phases, a lower stability of the additive, reflected by a lower melting temperature, may promote reactive processes to start earlier leading to coarsening of the grains belonging to secondary phases.     

MgB<Subscript>2</Subscript> with addition of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> obtained by spark plasma sintering technique

Superconducting bulks of MgB₂ with addition of Sb₂O₃ and Sb with different stoichiometric compositions ((MgB₂) + (Sb₂O₃) _x , x = 0.0025, 0.005, 0.015, and (MgB₂) + (Sb)_y, y = 0.01) were obtained by the Spark Plasma Sintering (SPS) technique. All added samples have high density, above 95% and critical temperature, T _c, of 38.1–38.6 K. This result and XRD data suggest that Sb does not enter the lattice of MgB₂. Impurity phases are Mg₃Sb₂, MgO, and MgB₄. The optimum addition is Sb₂O₃ for x = 0.005. This sample shows the critical current density, J _c(5 K, 0 T) = 4 × 10⁵ A/cm² and J _c(5 K, 7 T) = 6 × 10² A/cm², while the irreversibility field, H _irr (5 K, 100 A/cm²) = 8.23 T. Indicated values of J _c and H _irr are higher than for the pristine sample. The mechanism of J _c and H _irr increase in the Sb₂O₃ added samples is complex and composed of opposite effects most probably involving morphology elements, the presence of nano metric MgB₄ and the indirect influence of oxygen or oxygen and Sb. Crystallite size of MgB₂ is decreasing when Sb-based additions are introduced and the effect is stronger for the Sb-metal addition. The sample with Sb-metal addition does not improve J _c and H _irr when compared with pristine sample.     

Superconductivity in Fe and As Based Compounds: A Bridge Between MgB<Subscript>2</Subscript> and Cuprates

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multigap superconductivity, it is shown that these systems strongly resemble MgB₂, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similarly to MgB₂ and in contrast to cuprates, the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps.     

Phase formation of superconducting MgB<Subscript>2</Subscript> at ambient pressure

MgB₂ superconductor has been synthesized using a simple technique at ambient pressure. The synthesis was carried out in helium atmosphere over a wide range of temperatures. Magnesium was employed in excess to the stoichiometry to prevent the decomposition of MgB₂. Samples of MgB₂ thus prepared have been almost free from MgO as compared to other methods. Resistivities of the samples are quite low with residual resistivity ratio (RRR) of around 3.T _c (R = 0) is 38.2-38.5 K with ΔT _C of 0.6–1.0 K. Comparative studies of various methods of low pressure synthesis have been presented.     

Al-Doping Effects on the Structural Change of MgB<Subscript>2</Subscript>

A series of polycrystalline Al doped Mg_1−x Al _x B₂ (x=0.00, 0.01, 0.03, 0.06, 0.10, 0.15) samples were prepared using the solid-state reaction route. Phase analysis showed that Al is alloyed into the MgB₂ lattice and there were some Al₂O₃, MgAlB₄ particles present in bulk samples of MgB₂. It is shown that the suppression of T _c by doping originates largely from structural changes and the structure properties play an important role in influencing the normal-superconductor transport. The introduction of defects into the Mg layers and other aluminum compound (Al₂O₃, MgAlB₄) impurity phases both influence the polycrystalline structure.     

Nano-SiC Doped MgB<Subscript>2</Subscript> Bulks Synthesized by Rapid Process

Both pure and nano-SiC doped MgB₂ bulk samples were prepared by a rapid process. The effects of fast heating-up and doping on the critical current densities were studied. All samples were examined using XRD, SEM and magnetization measurement. Both pure and doped samples show higher critical current densities and irreversible field H _irr compared with samples sintered at the same temperature by a slow process. Experimental results manifest that fast heating-up is effective in reducing the grain size. This rapid process may be preferable to avoid grain size enlarging when element substitution can be achieved only at relatively high temperatures.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

Kinetics analysis for the sintering of bulk MgB<Subscript>2</Subscript> superconductor

The pertaining kinetic characteristics during the sintering of bulk polycrystalline MgB₂ superconductors is essentially important for the improvement of properties. Here Differential Thermal Analysis was adopted to record the heat effect during the preparation of bulk MgB₂ samples. The reaction between Mg and B powders starts before the melting point of pure Mg and the evolution for the fractions of MgB₂ were determined as a function of sintering temperatures. After fitting with different kinetic mechanism functions assumed, the sintering process of bulk MgB₂ superconductors was attributed to a solid-state interface-reaction controlled mechanism with an apparent activation energy of 4.54 × 10⁵ J mol⁻¹. Combined with microstructural observations by scanning electron microscopy and phase identification by X-ray diffraction, the formation process of MgB₂ phase was classified into two different stages: (i) solid-solid reaction stage, in which Mg and B powder starts to react and the growth of MgB₂ grain is restricted by the pinning effects of pores; (ii) solid–liquid reaction stage, in which the molten Mg melt promotes the reaction process and the regular hexagon bulk MgB₂ grain forms in a solution-reprecipitation and growth mode.     

Synthesis and Optical Studies of Superconducting MgB<Subscript>2</Subscript> Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)^1/2=β _T(E −E _g). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameterβ _Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Ac Susceptibility Measurements and Mechanical Performance of Bulk MgB<Subscript>2</Subscript>

The effects of Ar ambient pressure (vacuum and 0, 10, and 20 B) and annealing times (0.5 and 1 h) on microstructural, superconducting, and mechanical properties of bulk superconducting MgB₂ are investigated. The samples are produced using the solid-state reaction method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements are performed for determination of the crystal structure and surface morphology of MgB₂ samples, respectively. The superconducting properties are studied by AC magnetic susceptibility measurements. Microhardness analyses are made using the Vickers microhardness test for determination of mechanical properties of all samples. Increasing the Ar pressure decreases the lattice parameters and hence the average grain size. Increasing the annealing time results in larger lattice parameters and larger grain formation. The susceptibility measurements revealed a two-step transition which is reminiscent of granular superconductors. The intra-grain transition temperature is determined to be 38.4 K for all samples. The inter-grain transition temperature of 37.2 K is obtained for samples produced under Ar ambient pressure. The samples produced under Ar ambient pressure have better superconducting properties than the ones produced in vacuum. Increasing the annealing time under vacuum further decreases the superconducting properties probably due to Mg loss.     

Improved Superconducting Properties in Graphene-Doped MgB<Subscript>2</Subscript> Prepared by Coating Method

The effects of graphene doping on the phase formation and superconductivity of MgB₂ bulks synthesized with different process have been studied systemically. Considering the scattering structure of graphene, coating method was applied to enhance the uniformity of graphene doping. The graphene coated B addition was expected to improve the critical current density of MgB₂ bulks. In our study, several experiments were performed to find out the suitable way for graphene doping. The coating method could enhance the critical current density of MgB₂ from 1.9 × 10⁵ to 2.5 × 10⁵ A/cm² at 20 K and 0 T, compared with that of the undoped sample. And the superconductivity of MgB₂ prepared by coating method got obvious improvement at high field compared with that of pure graphene doping bulk. It can be concluded that the coating method could ensure the uniformity of graphene doping in MgB₂ and refined the grain crystalline effectively.